Network cameras for surveillance have come into widespread use. Various types of cameras are installed in various places, such as streets, public facilities, and houses. Some cameras are covered with dome-shaped covers. FIG. 10A is a schematic diagram illustrating a camera having a dome-shaped cover. The dome-shaped cover is illustrated in a cutaway view to show the camera disposed therein. A lens barrel 109 is capable of changing its orientation three-dimensionally in a dome-shaped cover 104, so that surveillance can be carried out in any selected shooting direction. A rotating direction 102 is referred to as ‘pan’, and a vertical tilting direction 101 is referred to as ‘tilt’. In general, the orientation of the camera is expressed in terms of pan and tile angles. The tilt angle is 0° when the camera is oriented horizontally. The tilt angle is 90° when the camera is oriented toward the zenith.
The dome-shaped cover has a function of protecting the camera from deterioration due to weather and malicious damage or defacement. The dome-shaped cover is an exterior component, and is also a portion of an optical system of the camera at the same time. FIG. 10B illustrates the overall structure of the dome-shaped cover. An optically effective region 105 having a hemispherical shape is required to have a shape accuracy as high as that of lenses. The hemispherical surface has an open end connected to a substantially cylindrical portion 106, and the substantially cylindrical portion has a flange-shaped collar portion 107. The substantially cylindrical portion 106 is shaped such that the inner volume of the space that accommodates the camera can be increased. The collar portion 7 is shaped such that the dome-shaped cover can be bonded to a cover barrel 108.
Dome-shaped covers are generally manufactured by plastic injection molding to achieve mass production. For example, a cup-shaped container or the like is generally formed by arranging a pin gate of a hot runner at the shape center and performing molding such that the flow length to the opening edge of the container, which is at the flow end, is uniform over the entire circumference to suppress molding irregularities.
However, the hemispherical optically effective region of the dome-shaped cover is required to have high shape accuracy, and the gate cannot be arranged in this region. When the gate is arranged in the optically effective region, a gate mark remains on an optical surface, and the imaging performance of the camera will be degraded. Therefore, the dome-shaped cover cannot be manufactured by a method similar to the method for manufacturing the cup-shaped container. As illustrated in FIG. 11, a side gate 112 is generally arranged at an end of the collar portion 107 instead of a shape center 114.
When the side-gate method is used, there are two major paths along which the resin flows to a collar end portion 113, which serves as a flow end portion, at an end opposite the gate-side end. One path is a path 116 that extends directly upward from the gate along the substantially cylindrical portion 106, passes through the zenith (shape center) 114 of the hemispherical portion 105, and reaches the collar end portion 113 at the end opposite the gate-side end. The other path is a path 115 that extends from the gate to the collar end portion 113 at the end opposite the gate-side end in the circumferential direction of the substantially cylindrical portion 106. When the lengths of these two paths are compared, the former path is longer by twice the height k of the substantially cylindrical portion. In other words, the flow through the zenith is slower than the flow in the circumferential direction. FIG. 12A is a front view of the dome viewed from the end opposite the gate-side end immediately before the injection of the resin is completed. FIG. 12B is a front view of the dome viewed from the end opposite the gate-side end immediately after the injection of the resin is completed. Since the flow in the circumferential direction is faster, flow fronts of the resin join along a linear line at the end opposite the gate-side end on the cylindrical portion (angle a is small). As the angle a decreases, the length of the linear region in which the flow fronts of the resin join increases, and a weld mark 117 more easily remains as a joint line. The weld mark 117 serves as molding irregularities that cause a reduction in the optical performance. As the magnification of the camera increases, the diameter of the dome increases, and the height k of the substantially cylindrical portion increases accordingly. As a result, the weld mark is more easily formed.
In PTL 1, the resin flow through the zenith is accelerated by increasing the thickness of the zenith portion. In addition, the resin flow in the circumferential direction is decelerated by forming a thin portion along the opening edge of the dome-shaped cover.